The long term rodent studies of 18 to 24 months duration are the accepted tests for evaluation of carcinogenic potential of chemicals. Historically, regulatory agencies of many countries accept 18 months as an adequate duration for carcinogenicity studies in mice. Recent guidelines, however, suggest longer studies. The duration of chronic toxicity and carcinogenicity studies with the B6C3F1 mouse for the National Toxicology Program and its predecessor, the National Cancer Institute, is 24 months. Little information is available concerning the advantages and complicating factors for conducting studies of longer than 24 month duration. The purpose of this study is to determine the age related physiological and pathological changes that may aid in the selection of study duration of toxicology and carcinogenesis studies in the B6C3F1 mouse.